The present invention relates to a cutting tool for metal machining with excellent hot hardness, wear resistance and toughness in which the binder phase is metallic tungsten or a tungsten alloy, a method of making the same and the use thereof. It is particularly useful for the machining of work materials with high hardness such as hardened steel.
Cemented carbide is a well known cutting tool material for metal machining and includes a large number of alloys of a binder phase and a hard phase, where the binder mainly comprises cobalt and the hard phase mainly comprises WC, possibly with additions of cubic carbides such as TiC, TaC, NbC etc or mixtures thereof. The manufacture of cutting tools from cemented carbide involves pressing of specially prepared powder followed by sintering at a temperature where the cobalt melts. The properties of the sintered material can be varied widely in terms of toughness and hardness depending on the amount of cobalt and hard phase. In use, the hot hardness of the material determines which temperature it can be subjected to without being plastically deformed.
This can be a serious limitation especially in the area of metal cutting, making the use of more exclusive and expensive materials such as ceramics, CBN and diamond necessary.
In attempts to improve the hot hardness, the obvious way is to substitute cobalt with a metal of high melting point. However, this creates new problems, since sintering temperatures need to be very high in order to melt the binder phase and is therefore not realistic for large scale production.
GB 504 522 discloses a method of manufacturing an alloy consisting of W or Mo with additions of Co, Si and B plus WC. Sintering is performed at a temperature below the melting point of W. A mixture according to the patent consisting of between 15 and 35% W is subjected to a pressure of 16.5 MPa at a temperature between 1750° C. and 1900° C. for 15 minutes. It is contended that the process produces an alloy with metallic W as binder.
GB 503 397 relates to a method for producing an alloy consisting of the same type of binder, i.e. W or Mo with additions like cobalt, silica and boron, but in this case cubic carbides TiC, MoC and TaC as hard phase. In this case, the suggested pressure was 23.4 MPa and the temperature 2500° C.
U.S. Pat. No. 3,507,631 disclose a material consisting of different nitrides as the hard phase using Mo, W, Rh or mixtures thereof as binder. The ratio between the thermal coefficient of expansion of the binder and the nitrides should be less than 2 and as a special case up to 50% of the nitride phase may be replaced by oxides, silicates and carbides of Ti, Nb, Zr and Ta. WC is not included as hard phase. Manufacture is by hot pressing at 1800° C.
SE 8406461-7 describes pros and cons for different ways to manufacture alloys consisting of carbides, nitrides or oxides of Ti, Zr, Hf, V, Nb, Cr, Mo or W and a binder consisting of one or more of the elements Ti, Zr, Hf, V, Nb, Cr, Mo or W. An example of a manufacturing method involves powder pressing, pre-sintering, final sintering and isostatic pressing. The binder phase content is relatively high, at least 25% by volume and as high as 70% by volume. It is also stated that performance in metal cutting is about five times better than the corresponding commercially available cemented carbide grade.
In an attempt to produce an alloy with 18% WC in a binder phase of W, it was proceeded according to the invention disclosed in the above mentioned GB 504 522. However, it was found that the process conditions suggested in the patent actually produced a brittle material with low hardness. It was specifically found that the process conditions suggested in the patent will actually produce an alloy where most of the W is transformed into W2C. This is not mentioned in the patent and it is reasonable to assume that limitations in the laboratory equipment at the time of invention failed to indicate this.